Upgrade Django from 5.2 to 6.0 (#20).

Align Model.save with keyword-only Django 6 API, fix recaptcha score widget usage, and make Payments.date use a callable default.

uv.lock

@@ -101,16 +101,16 @@ wheels = [
 
 [[package]]
 name = "django"
-version = "5.2.16"
+version = "6.0.7"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
     { name = "asgiref" },
     { name = "sqlparse" },
     { name = "tzdata", marker = "sys_platform == 'win32'" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/a9/26/889449d521ae508b26de715954faecd8bcf3f740affb81b2d146a83b42a5/django-5.2.16.tar.gz", hash = "sha256:59ea02020c3136fce14bef0bbece21a10a4febef5eed1c51c22ae468efa22200", size = 10890894, upload-time = "2026-07-07T13:52:17.005Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/89/55/664f24ff81c9ea19cb7dfc851afeae1f3c2390c7aee01d4ded68b5c1580d/django-6.0.7.tar.gz", hash = "sha256:2998503fc083124fb58037084bfa00de323c7c743f05f1b4284e77bff0ab8890", size = 10921299, upload-time = "2026-07-07T13:51:26.485Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/4e/13/1e5e3e4c15dcecb04281b3cb2a46a4670e1cef131068e202f6040df19224/django-5.2.16-py3-none-any.whl", hash = "sha256:04f354bf9d807a86ad1a8392fe3808d362358a8eafc322848e0e43e59b24371d", size = 8311943, upload-time = "2026-07-07T13:52:11.223Z" },
+    { url = "https://files.pythonhosted.org/packages/ba/ec/1ce5334b6a2c52ce619c23a0be8d366a57a0e080ebb2d88266e5c849157c/django-6.0.7-py3-none-any.whl", hash = "sha256:a037427c2288443a8c02a1b02295a31c239663aa682bc50b1976afb7cf6a769e", size = 8373344, upload-time = "2026-07-07T13:51:20.007Z" },
 ]
 
 [[package]]
@@ -366,7 +366,7 @@ dev = [
 
 [package.metadata]
 requires-dist = [
-    { name = "django", specifier = ">=5.2,<6" },
+    { name = "django", specifier = ">=6.0,<7" },
     { name = "django-phonenumber-field", specifier = ">=8.1.0" },
     { name = "django-recaptcha", specifier = ">=4.1.0" },
     { name = "gunicorn", specifier = ">=23.0.0" },